High pressure fluid flow systems need to be designed to resist significant operational stresses. An example of such a fluid flow system is a fuel injector for use in the delivery of fuel to a combustion space of an internal combustion engine. For heavy-duty applications, such as fuel injection for diesel engines for trucks, fuel injectors must be capable of delivering fuel in small quantities at very high pressures (of the order of 300 MPa).
Tensile stress is a significant cause of failure in such systems—cracks will be propagated by tensile stress but not by compressive stress. The intersection between two fluid bores has a significant failure risk associated with it in such a system, as it generally acts as a concentrator for tensile stress. In order to reduce the cost of products, it is also desirable to reduce material grade. This would usually reduce material strength, which can increase the failure risk at such intersections.
Such intersections will often be required in a design for a fuel injector. FIG. 1 shows an example of such a component stack used in such a fuel injector design. This fuel injector, discussed in full in European Patent Application No. 09168746.7, is discussed here to illustrate where such intersections may be required in such a design.
FIG. 1 shows a schematic view of a part of a fuel injector for use in delivering fuel to a combustion space of an internal combustion engine. The fuel injector comprises a valve needle 20 (shown in part) and a three way needle control valve (NCV) 10. The injector includes a guide body 12. The NCV 10 is housed within a valve housing 14 and a shim plate 16, which spaces apart the guide body 12 and the valve housing 14.
The valve needle 20 is operable by means of the NCV 10 to control fuel flow into an associated combustion space (not shown) through nozzle outlet openings. The lower part of the valve needle (not shown) terminates in a valve tip which is engageable with a valve needle seat so as to control fuel delivery through the outlet openings into the combustion space. An upper end of the valve needle 20 is located within a control chamber 18 defined within the injector body. This upper end slides within a guide bore 22 in the guide body 12 and acts as a piston. The control chamber 18 has two openings. One, at the top of the control chamber 18, leads to a first axial drilling 42 in the shim plate 16. The other, at the side of the control chamber 18, opens into a flow passage 52 in the guide body 12 that itself leads to a second axial drilling 44 in the shim plate 16. Both these axial drillings 42, 44 connect, through a cross slot 46, to a shim plate chamber 36 used for the NCV 10.
The NCV 10 controls the pressure of fuel within the control chamber 18. The NCV includes a valve pin with an upper guide portion 32a and a lower valve head portion 32b. The guide portion 32a slides within a guide bore 34 defined in a NCV housing 14. The valve head 32b slides within the chamber 36 between two valve seats 48, 50. High pressure fuel reaches the NCV 10 through a supply passage 30 extending through the guide body 12 and the shim plate 16, the supply passage 30 communicating with the NCV through a passage entering the guide bore 34 from the side. Fuel can leave the NCV through the cross slot 46 as discussed above or through a drain passage 38 communicating with a low pressure drain.
As previously stated, the NCV 10 controls the pressure in the control chamber 18 and hence movement of the valve needle 20. In one position of the NCV 10, fuel flows through the NCV 10 through the cross slot 46 and into the control chamber 18 to pressurise it, and in another position fuel cannot flow into the control chamber 18 but instead drains from it through to the cross slot 46 and hence to the drain 40. The specific details of this arrangement are described in more detail in European Patent Application No. 09168746.7.
The significance of the FIG. 1 arrangement to the teaching of this specification is that it illustrates the use of cross drillings in high-pressure injector designs. Two separate examples are shown: flow passage 52 is a cross drilling in the guide body 12 into the control chamber 18; and fuel supply 30 flows into guide bore 34 through a cross drilling in the valve housing 14. Both these cross drillings experience cycling between low and very high pressure, and are thus exposed to very high tensile stresses. This creates a significant risk of early component failure through crack propagation.
It is therefore desirable to protect components exposed to high tensile stresses against these stresses, and hence against fatigue limiting component life. The geometry of the intersection may be designed to reduce such stresses, but it is difficult to do this robustly and it will lead to increased production costs (both in machining and in process development). There are also conventional approaches that may be used to reduce net tensile stress by building in residual compressive stresses. Such processes include shot peening (in which a surface is bombarded with shot at a force sufficient to cause plastic deformation) and autofrettage (in which the chamber to be treated is subjected to exceptionally high pressure), but such processes are very expensive, may affect production processes and also may lead to robustness problems.
It is therefore desirable to prevent fatigue failure in regions of very high tensile stress, such as cross drillings into a main bore, without the problems of the prior art as discussed above.